1. Field of the Invention
The invention relates to the field of printing technology. It is based on a method of controlling the drive for transporting a paper web in a printing machine according to the preamble of the first claim.
2. Discussion of Background
The invention is particularly advantageously used for shaftless rotary printing machines. A shaftless rotary printing machine having blanket and plate or forme cylinders combined in pairs to form cylinder groups is described in DE 43 44 896 A1. A method of operating a drive system which is suitable, in particular, for shaftless rotary printing machines is described by DE 196 26 287 A1. According to this method, the torque control of the driven axles has superimposed on it speed control (rotational speed control) or position control (angular position controls). In this case, in order to control the individual drives for printing cylinders and folders under the stringent requirements for angularly synchronised running which rotary printing machines have to meet, use is preferably made of position controls.
A significant advantage of individually driven rotary printing machines is that these machines are capable of carrying out a product change while the machine is running. Such product changes include the throwing on and throwing off of printing points, as well as changes of paper graded while the machine is running, which may lead to considerable and impermissible changes in the paper web tensions.
During the operation of drive groups which are controlled in this way--and in particular when products changes are also carried out while the machine is running, or when changes of paper grade take place within continuous production--undesired subsidiary phenomena occur, such as excessively fluctuating web tensions and, as a result of this, for example paper breaks.
The conventional methods of controlling the drive for transporting a paper web primarily involve two modes of operation: the drives are controlled either to constant speed or to constant web tension. These methods have, in either form, the disadvantage that:
The paper draw between the threading mechanism and the printing unit builds up only very slowly when the machine is being started up. PA1 The web tension fluctuates to a relatively pronounced extent during normal operation. PA1 During a change of paper grade or of production sequence, the web tension fluctuates to an extremely pronounced extent, which can be attributed to the abruptly changed paper web lengths when throwing on or throwing off the printing units. This applies both when the web is guided over a plurality of printing towers and also when it is guided within one printing tower, a turning tower or the folder. PA1 The web tension is not maintained if the machine is stopped in an emergency. PA1 on the one hand, the paper properties of the running web change and, in the case of constant speed control, frequent and large-scale control compensation operations necessarily have to be carried out and PA1 on the other hand, the machine is not able to compensate for such control deviations in the desired time, from the point of view of control dynamics, and in addition has a tendency to an oscillatory behavior. PA1 In pre-controlled operation, high static accuracy of the web tension is achieved, this having a low sensitivity to interfering variables. PA1 Since the abovedescribed loading curve is part of the speed controller, the behavior exhibits extremely high dynamics. Characteristic reaction times of less than one millisecond are therefore possible. This is significantly faster than in the case of conventional web tension control. PA1 Furthermore, the loading curve leads to damping of mechanical oscillations. This property is important above all in the case of mechanical stimuli, such as those which occur when the paper grade or the production sequence is changed.
All these disadvantages can be attributed to the fact that